The present invention relates to a diesel engine for a vehicle, and particularly relates to a diesel engine with a comparatively low compression ratio in which its geometric compression ratio is set within a range of 12:1 to 15:1.
In diesel engines that perform combustions by fuel, in which a main component is diesel fuel being compressed to cause a self-ignition, a comparatively low compression ratio in which a geometric compression ratio is, for example, 15:1 or lower is set in order to perform, for example, a reduction of a discharge amount of NOx. That is, the low compression ratio subsides the combustion rate in the cylinders and suppresses generation of NOx (see JP2008-261236A). Because the lowered compression ratio of the engine reduces mechanical resistance, it is also effective in improving a thermal efficiency.
However, when the geometric compression ratio of the diesel engine is set low, a temperature of the engine at the end of a compression stroke is decreased corresponding to the low compression ratio. Therefore, self-ignition conditions become difficult to be met within, for example, an operation range where the engine has a low load and a low rotation speed. Further, particularly in the diesel engine disclosed in JP2008-261236A, an ignition performance degrades as a cetane number becomes lower depending on the property of fuel to be supplied. Therefore, when factors relating to the operating conditions of the engine and the property of the fuel are combined, the self-ignition conditions further become difficult to be met. Thus, surely securing an ignitability of fuel is important in setting the low compression ratio for the diesel engine regardless of the operating conditions of the engine and the property of the fuel to be supplied.